One of the characteristic parameters in describing the flow condition within a pneumatic conveyor conduit is the loading .mu., which indicates the relationship between the mass flow of bulk material, i.e. the through-put of material, and the mass flow of gas. In this connection, the conveying operation becomes more economical in proportion as the above-mentioned loading is increased, that is to say, as the weight of bulk material which is transported for example with one kilogram of air increases. Obviously, the pressure differential required for the pneumatic conveying operation also rises with increasing loading .mu.. At the same time, in order to avoid the necessity of using large amounts of conveyor air with the resulting disadvantages, and in order to keep wear and noise in the conveyor conduit and associated equipment to a low level, industrial practice is to attempt to operate with the lowest possible conveyor air velocity. It will be appreciated that, as this means that the conveyor pressures must accordingly be very high, problems arise with respect to the operation of introducing the bulk material to be transported from the atmosphere into the conveyor conduit in which there is a substantially increased pressure, and in metering and controlling such input of bulk material.
When operating with high conveyor pressures, for example a pressure of several bars, pressurized containers or pressure tanks are used, virtually without exception, for the pneumatic conveying operation. When such containers are employed as independent units, they suffer from the disadvantage that they can only be filled and then emptied, in succession. This discontinuous mode of operation, and the resulting dead periods which do not make any contribution to the conveying action, mean that the equipment must be of such dimensions that the through-put of bulk material during the active conveying phase is about 50 to 100% higher than its mean value in time, and pipes, compressors, separators and other fittings of the equipment suffer from a corresponding increase in size.
An improvement can be achieved in this respect, by using pressurized containers which are connected in parallel with each other, as in such an arrangement one container is filled while the other is emptied.
Arranging two pressurized containers in succession, one above the other, alternatively makes it possible to provide for a continuous pneumatic conveying operation. With this arrangement, firstly the upper container is filled. Subsequently, the filling opening of the upper container is closed and then the upper and lower containers are brought to the same pressure via a balancing conduit. The bulk material in the lower container can now be discharged into the conveyor conduit and conveyed pneumatically. After the outlet of the upper container has been closed again, the upper container is filled and the charging cycle is repeated.
If the two containers are reduced in size, use is often made of an arrangement known as a double-pendulum flap valve assembly. In this arrangement, the flaps are closed and opened alternately by a mechanical actuating means from the outside of the containers. The bulk material first falls from a storage container into a middle chamber which is subsequently discharged into a conveyor chamber which is under pressure. As the flaps must be opened against the pressure force which prevails below them, considerable actuating forces are required. In one construction, such forces are reduced by the closure flaps themselves not being fixedly connected to the actuating means. Instead, the actuating means is connected only to auxiliary flaps which can move upwardly into contact with the freely hanging main closure flaps, and thereby close openings in such flaps. The actual closure force for the main flaps is applied by the upwardly directed pressure differential. The actuating means is required to exert only a small force for opening the auxiliary flaps, as, when the auxiliary flaps open the openings in the main flap, there occurs pressure equalization between the chambers lying above and below the main flap, and the upper flap opens automatically, due to the force of gravity acting thereon.
However, this double flap assembly suffers from the disadvantage that it is actuated mechanically from the exterior, which means that rotary actuating members must be extended from the dust-bearing pressurized chamber into the atmosphere. In addition, it can happen that particles of bulk material may become trapped by the auxiliary flaps at the openings in the main flaps, so that the upper pressure chamber cannot be correctly sealed off. When that happens, an air-dust mixture can flow through the resulting gap, at a speed approximating the speed of sound. On the one hand this results in loss of air from the pressure chamber and on the other hand it also causes wear of the sealing surfaces, as by a sand blasting effect. In addition, most double pendulum flaps can only be used for conveyor pressures of up to about 2.5 bars.
One form of apparatus for the delivery of bulk material into a conveyor conduit, comprises a first chamber or antechamber whose inlet can be closed by a first closure flap and which has an outlet communicating with a second or conveyor chamber below the antechamber. A second flap is provided to close the outlet from the antechamber. The conveyor chamber has an outlet for connection to the conveyor conduit. The conveyor chamber is also connected to a conduit for supplying clean pressure air to the conveyor conduit for material conveying purposes, via a branch conduit. An air connecting conduit with a shut-off valve interconnects the antechamber and the conveyor chamber, while a vent with a vent control valve connects the antechamber to atmosphere. The apparatus also has a control means for alternate actuation of the shut-off valve and the vent valve. The first, upper, closure flap, when in an unloaded condition, only provides a narrow gap for the intake of the bulk material into the antechamber, and the lower, second, closure flap, which is in the form of a spring-loaded conical valve, does not open the antechamber outlet at all when in the unloaded condition, and only opens the antechamber outlet to the extent of providing an annular gap, when bulk material loads the conical valve. The small free cross-sectional areas produced by such valves permit only a low through-put of bulk material or make it necessary for the corresponding openings and the associated closure flaps to be of very large dimensions. Furthermore, this apparatus can easily suffer from operational trouble, if bulk material remains lying on one or both of the closure flaps, and thereby prevents the closure flap or flaps from completely closing. At high conveyor pressures, the bulk material then entrained by the flow of leakage air through the openings left by the incomplete closure of the flaps can then cause a weakening of the sealing action. In addition, in this apparatus, the various charging and venting valves are controlled by way of a gear assembly, which means that a given dead time must necessarily be provided in the operating cycle, in order to ensure that the conveyor chamber is always completely emptied before a fresh filling operation, as otherwise the conveyor chamber could be overfilled. The apparatus can therefore not operate with the optimum through-put of bulk material.